Vertically pivoting ramp doors are common features on cargo trailers, as well as, various types of recreational vehicles and enclosures. Ramp doors are hinged to cargo trailers to allow the ramp doors to swing from a vertical closed position where the ramp door encloses the door opening to an open position where the ramp door contacts the ground, thereby providing a ramp for access to the trailer interior. Powered ramp door lifts used to automatically raise and lower the ramp doors are a highly desirable feature, particularly for applications with large heavy ramp doors. Typically, powered ramp door lifts consist of one or more cables connected to the ramp door and a simple cable winch mounted within the trailer interior. While, simple cable winches winding a single cable can be used to lift the ramp doors in a ramp door application, two cables are needed to ensure that the ramp door remains level as it moves between the open and closed position and prevent the twisting of the ramp door under its own weight.
Heretofore, conventional powered lifts have been used in cargo trailers and other applications with limited success. One of the major drawbacks of conventional lifts in ramp door applications has been the tendency of the cables to “unspool” from the webs of the winch spool in the absence of cable tension from the winch. When two cables are used to support the ramp door, the tension on each cable is often unequal. This difference in tension may be the product of a variety of practical factors, including: the difference in the length of cable wound around each spool, differences in the amount of cable stretch, variances in the outer diameters of the spools and/or the manner in which each cable is wound onto the spools. Any differences in cable tension between the two cables results in a slack cable, which leads to unspooling problems. Consequently, greater care must be taken to install dual cable powered lifts to ensure little difference in the cable tension between the two cables.
In addition, when the ramp door is in the vertical closed position, the weight of the door rests directly over the door hinge, such that the force of gravity acting on the door in the vertical plane is substantially opposite the supportive force of the hinge. Due to the mass of the ramp door and its static rotational inertia in the vertical closed position, the ramp door tends to remain at rest as the winch begins to unwind the cables. While cable tension provided by the winch holds the ramp door in the vertical closed position, if the ramp door does not immediately begin to lower with the unwinding of the winch, cable tension is lost. In the absence of cable tension, the resilient memory of the cables will cause the cables to “unspool” and become tangled or bound in the winch apparatus. Users often must pull on the closed ramp door to over come the initial rotational inertia of the door as the winch unwinds the cable. To pull the ramp door open, the user must often be positioned under the lowering ramp door, which creates a potentially hazardous situation. Conventional powered ramp door lifts using simple cable winches, like torsion spring counterbalances that are used to unweight the ramp doors in manually lifted ramp door applications, only exert a force on the door through the cables to pull the door closed. In the vertical closed position, the force applied to the ramp door by the lift is inward. Consequently, a powered ramp door lift that provides a small outward force to the ramp door to maintain cable tension during the initial travel of the ramp door from the closed position is needed to ensure consistent and instantaneous operation of the lift in raising and lowering the ramp doors between the closed and open positions.